Here is roughly the route they plan to take. 7,669 km, assuming the 3.9L/100km fuel efficiency Wikipedia reports, should take 299L of fuel. It's not bad, but that's 300L of fuel that wouldn't be burned if they stayed at home. Meanwhile we're not taking into account travel around the cities they visit, the two ferry rides they need to take, and of course the fact that you never get remotely close to published fuel efficiency ratings.